Immune response and other parameters of malarial infection were studied in mice given either nonlethal Plasmodium berghei yoelii (17XNL) or a lethal variant (17XL). Infection with 17XL led to rapid parasitemia and mortality of greater than 90% by 9 days. In vitro T lymphocyte proliferation of spleen cells was much less when cells came from mice infected with 17XL than 17XNL. By contrast, similar sensitization was seen in lymph node cells. During the course of infection with 17XL, and early in the course of infection with 17XNL, immune responses in vitro were reduced to many antigens such as sheep and horse red blood cells and TNP-Ficoll, and proliferative responses to mitogens such as concanavalin and phytohemagglutinin. These suppressed responses seem to be mediated by macrophages. BIBLIOGRAPHIC REFERENCES: Weinbaum, F.L., Evans, C.B. and Tigelaar, R.E.: Immunity to plasmodium berghei yoelii in mice. 1. The course of infection in T cell and B cell deficient mice. J. Immunol. 117: 1999, 1976.